1. Field of the Invention
This invention relates to an ink-jet color recording process in which a plurality of colored pigment-based inks are used, and further to an ink set suitable for use in such process.
2. Related Background Art
In recent years, ink-jet recording systems have made rapid progress for office use as well as for personal use because they are less noisy, economical of running cost, easy to be colorable and feasible with downsized apparatus.
A keen demand has lately been voiced for ink-jet recording devices that are inexpensive in particular and capable of producing color images. In order to cope with this trend, it has been proposed to mount on such a type of device a plurality of recording heads each designed to provide the same ejection characteristics and also a plurality of recording liquids formulated with water-soluble dyes, thereby obtaining a color image. In case of color images, however, if a plurality of colors involves even one having insufficient fastnesses to water and to light, the whole color image often poses an extreme decline in quality. To eliminate this problem, it is desired that pigments which have a good fastness be employed as colorants for ink-jet recording liquids. There have been found various patent publications to that effect.
Almost all of the above publications are directed toward an improvement in preventing an occurrence of agglomeration, precipitation and separation of a pigment during storage of the resulting ink, or in avoiding a nozzle clogging due to the use of pigments. This means that less is elucidated as to the ejection characteristics of pigment-based inks relative to the associated recording heads. These characteristics here are typified by ejection quantity, ejection speed and frequency response and taken as essentially basic and most important ink-jet recording.
With respect to ink-jet color recording in particular, a limited coverage of technological findings is available at present. For instance, Japanese Patent Application Laid-Open No. 4-170474 proposes to improve an ejection stability, i.e., a durability of printing operation. To provide a color image of high quality, the first key resides in satisfying a given set of ejection characteristics, followed by retention of the ejection characteristics. With conventional dye-based inks, the ejection characteristics are easy to meet at an initial stage. On the contrary, in case of pigment-based inks, these characteristics vary largely with the kind of dispersing media used for pigments. This variation is in most cases dominated by a frequency response. In case of dye-based inks, a similar problem can be solved by the addition of various solvents and other additives since a dye is more stable in a liquid medium than a pigment, and hence, only modified formulations of the dye-based inks are sufficient to clear up that problem.
A pigment, however, is known to be poor in stability, namely dispersion stability, in a liquid medium as compared to a dye. Therefore, a pigment-based ink has a smaller amount of leeway in selecting solvents and additives than a dye-based ink and necessarily relies upon optimum ejection characteristics. In such instance, a single recording head may be optimized from the ejection viewpoint for ink-jet monochrome recording, whereas at least a few recording heads are required to be individually optimumly designed for ink-jet color recording.